“Loss of Astro Darkness” is not “a Thing” if your LP is more than a certain amount

[Captain Scarlet]

Summary: If you have more than a particular amount of Light Pollution, you will not lose astro darkness because you never achieve it anyway. Under LP, you reach your maximum darkness before the Sun gets to 18 degrees down. For example Hampstead in London never gets darker than about 18 mag/sq-arcsec which is reached with the Sun only about 10 degrees over the horizon. Any further sinking of the Sun makes no difference. Since the Sun does sink below 10 degrees down even at midsummer, Hampstead does not in fact ever lose maximum darkness. Hampstead would only lose maximum darkness if the Sun’s nadir was less than 10 degrees down (the value 18 mags/sq-arcsec as Hamsptead’s level of LP is read off from lightpollutionmap.info’s World Atlas 2015 layer which agrees very well with the data collected for the chart below).

 

More reasoned: In the Northern hemisphere around this time of year, many of us start to rue the upcoming “loss of astro darkness”. Astro Darkness is defined as the period after which the Sun dips below 18 degrees below the horizon, following the three phases of Twilight (Civil Twilight starts at sunset ending with the Sun 6 degrees down, Nautical Twilight is 6-12 degrees down and Astronomical Twilight is 12-18 degrees down). “Why 18 degrees?” you might ask. Is this value defined or fundamental?”. It’s fundamental, as can the chart below hints. Any lower than -18 degrees makes no further difference to how dark it can get.

Depending on your Latitude, the “nadir formula” is Sun_Nadir = 90 minus (observer’s latitude plus earth’s axial tilt). So at 51.5 N, where I live, the Sun’s nadir is 90 – (51.5 + 23.5) => 15 degrees down only, or mid-astronomical twilight. The chart (red points) shows therefore that at and around mid-summer I DO lose astronomical darkness: I can expect it to get no darker than around 21.5 mags for -15 degrees, whereas I should expect 22 mags at other times of year.

The further North you are, the worse this gets, both in terms of how far from astro darkness it gets, and the period for which it persists. My cousin who lives in Kuusamo, Finland at 66 degrees N has a local level of LP of 20.5 . At nadir, the Sun only gets to 0.5 degrees down! The chart “turning point” for LP of 20.5 max darkness of LP is reached with the Sun at -13 degrees. So she certainly loses maximum darkness! (for other latitudes see the table at bottom).

HOWEVER, it’s not quite that simple. Your local level of Light Pollution also has something to say. Consider the chart, plotting my own readings of darkness-level at zenith vs Sun altitude, using a Unihedron SQM-L, from two locations at which I have lived, over several years.

You may recognise it, I’ve wheeled it out in other contexts. One location, the blue points, is Bortle 7-8, 30km from the centre of London. The other, the red points, is Bortle 2-3, at the tip of South West Ireland. The wide scattering of many of the points well below the “darkest level” represents readings taken with the Moon up at various altitudes and phases.

Several observations follow:

-    As the Sun sinks, gradually losing altitude below the horizon, LP doesn’t seem to affect the zenith darkness value until you get close to your level of LP. The blue and red points follow the same path. IE until you’re within 2-ish Sun-degrees of your maximal darkness imposed by your LP level, it (twilight) will be equally dark wherever you are for a given altitude of the Sun.

-    For a dark location, 18 degrees down does indeed seem to be the altitude at which further depression of the Sun makes no difference to darkness. L-18 degrees is the “turning point”.

-    At the darker site, the midsummer Solar nadir of -15 degrees will yield indeed a brighter sky than -18 degrees, to the tune of approximately 0.5 magnitudes: astro darkness is lost.

-    At the brighter Bortle 7-8 location, affected by LP, the “turning point” is no longer -18 degrees. The chart shows it’s more like -12 degrees. IE the Sun-altitude at which it no longer gets darker is -12 degrees. So astro- (or more accurately “maximum-”) darkness is not in fact lost.

-    So if, for example you live and observe in the centre of London, you can be reassured that your darkest sky will be no less dark than at any other time of year. Sure, it’ll be shorter, but that’s just Summer.

Finally, here is a table showing my estimates of the Sun_Altitude turning-points for different levels of max darkness, and whether at various combinations of LP and Latitude, you can expect to lose your maximum darkness or not.

Thanks for reading, Magnus

 

Edited May 3 by Captain Scarlet

[TiffsAndAstro]

After reading this, I almost like light pollution. Almost.

[mackiedlm]

Very interesting and clearly explained.

 

Thal you

 

[Pete Presland]

Interesting read Magnus, i guess if you live where its really bad you don't even worry about the loss of the dark in the summer

the light pollution in Bedfordshire (Biggleswade) feels like it has got much worse in recent years.

Edited May 3 by Pete Presland

[DaveS]

The graph (Which I've seen before in your earlier post) and table validates my own empirical eyeball observation that below a Sun altitude of -16.5 degrees my sky doesn't become noticeably darker. My sky is a nominal SQI 21.66.

[david_taurus83]

I have imaged on 24th of June before, below pic was just over 2 hours worth from bortle 6 with a DSLR!

I just hope the longer days bring some weather with them!

 

[niallk]

Great post - very interesting!👍